test(rel,exec): isolate the differential fixed-hop reference implementation

[DecisionNerd]

Split from #1020 after architecture review.

Scope

relational_fixed_hop_reference is a differential-testing oracle reachable through production graphforge-rel and graphforge-exec APIs, although its known consumer is the differential traversal test.

Acceptance criteria

• Keep the oracle available to differential tests
• Remove it from ordinary production builds using a test-only crate/module or an explicit non-default test feature
• Prove the optimized and reference implementations remain covered by the differential suite
• Ordinary workspace builds expose no reference-mode switch